The present invention relates generally to the field of microwave devices and, more particularly, to suspended substrate circuits. Still more particularly, the present invention relates to rat-race couplers.
The hybrid ring circuit, also known as the rat-race coupler, has been used for many years and is still an essential part of many complex microwave circuits such as mixers, phase shifters and power dividers. The rat-race coupler has three of the four transmission lines between ports equal to one-quarter wavelength and one line between ports equal to three-quarter wavelengths at the midband operating frequency of the device.
Referring to FIG. 1 a prior art rat-race coupler is schematically illustrated. As can be seen in FIG. 1, the rat-race coupler 12 is comprised of a circular ring 14 and four conductive arms or ports 16, 18, 20 and 22. The arms 16 and 18 are separated by an electrical length of .lambda./4 where .lambda. is the wavelength at the midband operating frequency of the device.
Similarly, ports 18, 20 and 22 are separated, respectively, by an electrical length of .lambda./4. Ports 16 and 22 are separated by an electrical length of 3.lambda./4. It can readily be appreciated that an input on port 18 will result in an equal, in-phase division of the input signal on port 18 between ports 16 and 20. The port 22 is isolated from port 18 by reason of cancellation of the signals that would propagate around ring 14 in opposite directions. Particularly, any signal entering ring 14 via port 18 and propagating along the direction indicated by arrow 24 would propagate along an electrical length of .lambda. and any signal entering ring 14 via port 18 propagating in the direction indicated by arrow 26 along the ring would propagate through an electrical length of .lambda./2 to port 22. The propagation distance between port 18 and 22 via the direction of arrow 24 is therefore 180.degree. out of phase with the signal propagating along ring 14 via the direction of arrow 26 and, therefore, cancellation of these two signals occurs (assuming equal loads on ports 16 and 20). Thus, by inputting a signal on input& port 18, and using appropriate loads on ports 16 and 20, the rat-race 12 acts as a splitter, with all the input power: absorbed by the two loads. The rat-race can also be used as a mixer if, for example, a local oscillator (L.0.) signal is injected on port 22. With an L.0. signal injected on port 22, equal, 180.degree. phase shifted signals will appear on ports 16 and 20 and port 18 will be isolated. This is because the electrical propagation distance between ports 20 and 22 is .lambda./4 and the electrical propagation distance between ports 16 and 22 is 3.lambda./4. Diodes at ports 16 and 20 appear as loads to both the L.O. and input radio frequency (R.F.) signals. Often the resulting intermediate frequency (I.F.) is removed at the symmetric position 27.
Cutoff frequency is defined as the lowest frequency which can propagate through a guide in a waveguide mode. A below cutoff frequency waveguide or below cutoff waveguide is a guide having dimensions which will not allow waveguide propagation modes at frequencies below the cutoff frequency. Below cutoff waveguide housings are always used with suspended substrate circuits and often with millimeter wave microstrip circuits. In these applications, rat-race couplers cannot be used without widening the enclosing channel and providing mode suppresion pins or other mode suppression devices. The provision of mode suppression devices usually requires drilling holes in the circuit substrates.
One method of coupling both R. F. signals and L. O. power into two diodes is through the use of a rat-race mixer as described above with respect to FIG. 1. The use of two diodes in a balanced arrangement helps to cancel noise due to L. 0. sidebands. The rat-race mixer involves the use of a circular ring of about one-half wavelength in diameter. The width of the ring, e.g. the width of ring 14 in FIG. 1, and the addition of the four conductor arms, e.g. 16, 18, 20 and 22 in FIG. 1, to the hybrid ring does not allow its use in a suspended substrate housing. This is because the suspended substrate housing must have a channel width less than one-half the wavelength of the highest propagating frequency in order to prevent propagation in the waveguide TE.sub.10 mode.
FIG. 2 is a schematic illustration of a stripline channel 24 within a metallic housing and having a suspended substrate conductor 26 formed on a suspended substrate card (not shown) positioned within the stripline channel 24. In this prior art configuration TEM mode electromagnetic energy can propagate within the below cutoff waveguide 24. FIG. 3 is, likewise, a schematic illustration of a below cutoff waveguide channel 28 having a suspended substrate conductor 30 extending therethrough and also having a below cutoff waveguide channel 32 extending orthogonally from the primary channel 28. This configuration also permits the propagation of TEM mode energy within the below cutoff channels 28 and 32. FIG. 4 illustrates schematically two crossed below cutoff waveguides or channels 34 and 36 and having a suspended substrate rat-race coupler 38 positioned within the intersection of the crossed channels 34 and 36. It has been discovered that a suspended substrate transmission line within a housing having two crossed channels as shown in FIG. 4 will suffer extreme circuit loses. It is believed that the energy losses are due to higher order modes which occur when there are no sidewalls in each of the guides 34 and 36. The lack of one sidewall, as would be the case for the suspended substrate configuration of FIG. 3, does not cause any appreciable signal loss, however. The failure of the device constructed as shown in FIG. 4 is also believed to be attributable to the fact the sole sidewall current flow is necessary in order to support TEM propagation.